1. Field of the Invention
The present invention relates to an electronic display formed by fabricating an EL (electro luminescence) on a substrate, in particular, to an EL display using a semiconductor element (an element which uses a semiconductor thin film). Further, the present invention relates to a light emitting device using an EL display in a display portion.
2. Description of the Related Art
Recently, a technique for forming TFTs on a substrate has been remarkably developed, and its application to an active matrix electronic display has been in proceeding. In particular, TFTs using a polysilicon film can operate at high speed, because such TFTs have a higher field effect mobility than TFTs using a conventional amorphous silicon film. Therefore, the control of pixels, which has been conventionally conducted by a driver circuit provided outside a substrate, can be performed by a driver circuit provided on the same substrate on which the pixels are provided.
Such an active matrix electronic display includes various circuits and elements formed on the same substrate. With this structure, the active matrix electronic display provides various advantages such as reduced manufacturing cost, reduced size of an electronic display, an increased yield, and an increased throughput.
Furthermore, an active matrix EL display including an EL element as a self-light emitting element has been actively studied. The EL display is also called Organic EL Display (OELD) or Organic Light Emitting Diode (OLED).
In contrast with the liquid crystal display device, the EL display has a type of self-light emitting. The EL element has such a structure that a layer containing an organic compound (hereinafter, referred to as an EL layer) is sandwiched between a pair of electrodes (anode and cathode). The EL layer generates luminescence by applying an electric field across the pair of electrodes and has normally a laminate structure. As a typical example of the laminate structures, a laminate structure xe2x80x9chole transporting layer/light emitting layer/electron transporting layerxe2x80x9d proposed by Tang et al. of Eastman Kodak Company is cited. This structure has an extremely high light emitting efficiency. For this advantage, most light emitting devices, which are currently under study and development, employ this structure.
Furthermore, the light emitting device may have such a laminate structure that a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer, or a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer/an electron injecting Layer are deposited on an anode in this order. Moreover, the light emitting layer may be doped with a fluorescent pigment or the like.
All layers formed between a cathode and an anode are referred to generically as EL layers within this specification. The above stated hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer, and the like are therefore all contained within the EL layer. There is emission of light when returning to a base state from a triplet excitation state (phosphorescence), and when returning to a base state from a singlet excitation state (fluorescence), in luminescence of organic compounds, and a light emitting device of the present invention may use any one of the above light emitting types, or may also use both types of light emission.
A predetermined voltage is then applied to the EL layer with the above structure by a pair of electrodes, recombination of carriers thus occurs in the light emitting layer, and light is emitted. Note that the emission of light by the EL element is referred to as driving the EL element throughout this specification. Further, a light emitting element formed by an anode, an EL layer, and a cathode is referred to as an EL element throughout this specification.
As a method of driving an EL display, an analog driving method (analog drive) can be given. The analog drive of an EL display is described with reference to FIGS. 25 and 26.
FIG. 25 shows a structure of a pixel portion 1800 of an EL display that is driven in an analog manner. Gate signal lines (G1 through Gy) to which gate signals from a gate signal line driver circuit are input are connected to gate electrodes of switching TFTs 1801 included in respective pixels. Source regions or drain regions of the switching TFTs 1801 included in the respective pixels is connected to source signal lines (also referred to as data signal lines) S1 to Sx to which an analog video signal is input, whereas the others are connected gate electrodes of EL driver TFTs 1804 and capacitors 1808 included in the respective pixels.
Source region of the EL driver TFTs 1804 included in pixels are connected to power source supply lines Vd1 through Vx, whereas drain regions of the EL driver TFTs 1804 are connected to EL elements 1806. An electric potential of the power source supply lines V1 through Vx is referred to as an power source electric potential. The power source supply lines V1 through Vx are connected to the capacitors 1808 included in the respective pixels.
The EL element 1806 includes an anode, a cathode and an EL layer sandwiched between the anode and the cathode. If the anode of the EL element 1806 is connected to the drain region of the EL driver TFT 1804, the anode and the cathode of the EL element 1806 become a pixel electrode and an opposing electrode, respectively. On the other hand, if the cathode of the EL element 1806 is connected to the drain region of the EL driver TFT 1804, the anode and the cathode of the EL element 1806 become an opposing electrode and a pixel electrode, respectively.
Note that the electric potential of the opposing electrode is referred to as an opposing electric potential in this specification. Note also that an power source for imparting the opposing electric potential to the opposing electrode is referred to as an opposing electric power supply. The electric potential difference between the electric potential of the pixel electrode and the electric potential of the opposing electrode is an EL driver voltage, and the EL driver voltage is applied to the EL layer.
FIG. 26 shows a timing chart in the case where the EL display shown in FIG. 25 is driven in an analog manner. The period from the selection of one gate signal line to the selection of the next gate signal line is called one line period (L). The period from starting display of one image to starting display the next image corresponds to one frame period (F). In the case of the EL display shown in FIG. 25, since there are y gate signal lines, y line periods (L1 to Ly) are provided within one frame period.
With the enhancement in resolution, the number of line periods within one frame period increases. As a result, the driver circuit must be driven at a high frequency.
An power source electric potential at the power source supply lines V1 through Vx is held constant, and an opposing electric potential at the opposing electrodes is also held constant. There is a potential difference between the opposing electric potential and the power source electric potential to such a degree that a EL element emits light.
The gate signal line G1 is selected in the first line period L1 by a gate signal input to the gate signal line G1 from the gate signal line driver circuit.
Note that, in this specification, the term xe2x80x9ca gate signal line is selectedxe2x80x9d refers to a state in which all of the thin film transistors which have gate electrodes connected to the gate signal line are in an ON state.
An analog video signal is then input to the source signal lines S1 to Sx in order. All of the switching TFTs 1801 connected to the gate signal line G1 are in an ON state, and therefore the analog video signals input to the source signal lines S1 to Sx are input to gate electrodes of the EL driver TFTs 1804 through the switching TFTs 1801.
The amount of current flowing through a channel formation region of the EL driver TFT 1804 is controlled by a level of an electric potential (voltage) of a signal input to the gate electrode of the EL driver TFT 1804. Accordingly, the electric potential applied to the pixel electrode of the EL element 1806 is determined by the level of the electric potential of the analog video signal input to the gate electrode of the EL driver TFT 1804. Then, the EL element 1806 is controlled by the electric potential of the analog video signal to emit light.
When the above-described operation is repeated to complete the input of analog video signals to the source signal lines (S1 through Sx), the first line period (L1) terminates. One line period may alternatively be constituted by the period until the completion of input of the analog video signals to the source signal lines (S1 through Sx) and a horizontal blanking period.
Then, a second line period (L2) starts when a gate signal line G2 is selected by a gate signal. As in the first line period (L1), analog video signals are sequentially input to the source signal lines (S1 through Sx) during the second line period.
When all gate signal lines (G1 through Gy) are selected in this manner, all line periods (L1 through Ly) are completed. The completion of all the line periods (L1 through Ly) corresponds to the completion of one frame period. All pixels perform display during one frame period to form an image. One frame period may be alternatively constituted by all line periods (L1 through Ly) and a vertical blanking period.
The amount of light emitted by the EL element is thus controlled in accordance with the analog video signal, and gray scale display is performed by controlling the amount of light emitted. This method is namely a driving method referred to as an analog driving method, gray scale display is performed by changing the electric potential of the analog video signal input to the source signal lines.
The control of the amount of current to be supplied to an EL element by a gate voltage of an EL driver TFT in an analog driving method as described above will be described in detail with reference to FIGS. 27A and 27B.
FIG. 27A is a graph showing a transistor characteristic of an EL driver TFT. In this graph, a line 2801 is referred to as an IDS-VGS characteristic (or an IDS-VGS curve). Here, IDS indicates a drain current, and VGS indicates a voltage (gate voltage) between a gate electrode and a source region. This graph allows to see the amount of current that flows at an arbitrary gate voltage.
When gray scale display is performed in the analog driving method, the EL element is driven using a region shown by a dotted line 2802 of the above IDS-VGS characteristic. An enlargement of the region surrounded by reference numeral 2802 is shown in FIG. 27B.
A region shown by slanted lines in FIG. 27B is referred to as a saturated region. Specifically, if the threshold voltage value is taken as VTH, the saturation region is a region in which the gate voltage satisfies |VGSxe2x88x92Vth| less than |VDS|, and the drain current changes exponentially with respect to changes in the gate voltage in this region. Electric current control is performed by the gate voltage using this region.
A gate voltage of an EL driver TFT is determined by an analog video signal which is input into a pixel with turning on a switching TFT. At this time, based on the IDS-VGS characteristic shown in FIG. 27A, a drain current with respect to a gate voltage is determined in a one-to-one correspondence. More specifically, a voltage of the analog video signal input to the gate electrode of the EL driver TFT determines an electric potential of the drain region. As a result, a predetermined amount of drain current flows into an EL element so that the EL element emits light in the amount corresponding to the amount of the current.
As described above, the amount of light emitted from the EL element is controlled by a video signal to perform gray-scale display.
However, the above analog driving has a disadvantage of being extremely affected by fluctuation in TFT characteristics. Even when the same gate voltage is applied to the EL driver TFTs of the respective pixels, the EL driver TFTs cannot output the same drain current if there exists a fluctuation in IDS-VGS characteristics of the EL driver TFTs. Furthermore, as is apparent from FIG. 27A, since the region in which a drain current exponentially changes with respect to a change in gate voltage is used, the amount of current to be output may greatly differ with a slight shift in IDS-VGS characteristic even when the same gate voltage is applied to the TFTs. Under such a condition, even when signals of the same voltage are input, the amounts of light emitted from the EL elements of adjacent pixels differ from each other due to a slight fluctuation in IDS-VGS characteristic.
As described above, the analog driving is extremely sensitive to fluctuation in characteristic of EL driver TFTs, which is a problem in the gray-scale display of a conventional active matrix EL device.
The present invention has been made in view of the above problems, and has an object of providing an active matrix EL display which enable clear multi-gray scale color display. Moreover, the present invention has another object of providing light emitting devices (electronic appliances) with high performance, including such an active matrix EL display as a display.
The inventors of the present invention consider that the problems in analog driving result from conducting a gray-scale display using a saturated region that is susceptible to the effect of fluctuation in IDS-VGS characteristic due to an exponential change in a drain current with respect to a change in a gate voltage.
More specifically, in the case where there is a variation in IDS-VGS characteristic, a drain current exponentially varies with a change in a gate voltage in a saturated region. Therefore, even if the same gate voltage is applied, different currents (drain currents) are output, resulting in inconvenience that a desired gray-scale cannot be obtained.
Thus, the present inventors considered controlling the amount of light emitted from an EL element by control of a period during which the EL element emits light, not by control of current using a saturated region. In other words, the amount of light emitted from the EL element is controlled in terms of time to realize a gray-scale display in the present invention. A driving method for performing gray-scale display by controlling a light emitting period of an EL element is called a time-division driving method (hereinafter, referred to as digital driving). Gray-scale display realized by a time-division driving method is called time division gray-scale display.
In accordance with the above structure, even if there is some fluctuation in the IDS-VGS characteristic of the EL driver TFT, it becomes possible to avoid a situation in which the amount of light emitted from the EL elements differs greatly in adjacent pixels when signals of the same voltage are input with the present invention.